Phys Chem Minerals (1997) 24:488–494 C Springer-Verlag 1997 ORIGINAL PAPER A. Kubo  T. Suzuki  M. Akaogi High pressure phase equilibria in the system CaTiO 3 -CaSiO 3 : stability of perovskite solid solutions Received: June 17, 1996  Revised, accepted: October 22, 1996 Abstract Phase relations in the system CaTiO 3 -CaSiO 3 were experimentally examined at 5.3–14.7 GPa and 1200–1600 °C with a 6–8 type multianvil apparatus. As pressure increases, stability field of perovskite solid so- lution extends from CaTiO 3 to CaSiO 3 , and the per- ovskite becomes stable for the entire composition range above about 12.3 GPa. The stability field of Ca(Ti 1-X , Si X ) 2 O 5 (0.78x1) titanite solid solution +Ca 2 SiO 4 larnite exists in the CaSiO 3 -rich composition range at 9.3–12.3 GPa and 1200 °C. Perovskite solid solutions containing CaSiO 3 component of 0 to 66 mol% could be quenched to 1 atm. The composition-molar volume rela- tionship of perovskite solid solution showed that molar volume of perovskite solid solution linearly reduces from the value of CaTiO 3 to that of CaSiO 3 . Introduction CaSiO 3 perovskite, stable only at high pressure, is gener- ally agreed to be a minor phase in the Earth’s lower mantle. Because CaSiO 3 perovskite cannot be quenched to 1 atm, high pressure experiments on CaSiO 3 per- ovskite have been performed mainly by in situ methods. Ringwood and Major (1971) reported that perovskite solid solutions in the system CaTiO 3 -CaSiO 3 were quenchable to 1 atm for the compositions up to 83 mol% CaSiO 3 . However, phase relations in the system CaTiO 3 -CaSiO 3 have not been examined in detail. When these perovskite solid solutions are quenched to 1 atm in a wide range of composition, some physical prop- erties, such as elastic and thermodynamic properties, of CaSiO 3 perovskite could be estimated by extrapolation of the measured properties of these solid solutions in the atmospheric conditions. In this study, phase relations in the system CaTiO 3 - CaSiO 3 have been investigated at pressures of 5.3 to 14.7 GPa and temperature of 1200–1600 °C to determine the extent of perovskite solid solution. The relationship be- tween composition and molar volume of perovskite solid solution has also been determined. Experimental Several kinds of starting materials in the system CaTiO 3 -CaSiO 3 were prepared using reagent grade chemicals. Glasses with Ca- SiO 3 component of 65, 80, 90, and 95 mol% were synthesized from silicic acid (SiO 2  11wt%H 2 O), TiO 2 and CaCO 3 . These materials were thoroughly mixed and decarbonated at 850–950 °C for 1 hr and melted in a platinum crucible in air at 1700 °C. After being held at 1700 °C above the liquidus for about 1–2 h, they were quenched to glass in cold water. CaSiO 3 glass was also synthesized from CaCO 3 and silicic acid by a similar method. All of the glasses were confirmed to have compositions within +1% of nominal compositions by EPMA analysis. However it was difficult to pre- pare glasses which contained less than 65 mol% of CaSiO 3 compo- nent because of rapid nucleation of CaTiO 3 crystals during quenching. A mixture of fine-grained powder with the mol ratio of CaTiO 3 :CaSiO 3 =40:60 was also prepared by standard gel prepa- ration method, using CaCO 3 , titanium trichloride and tetraethy- lorthosilicate (TEOS). CaCO 3 dissolved into nitric acid was mixed with titanium trichloride and TEOS, and added with ammonia solution to precipitate gel. Then the gel was heated up to 600 °C and held for 92 hr. The synthesized mixture is called “gel” in this study. CaSiO 3 wollastonite was synthesized from silicic acid and Ca- CO 3 . These materials were mixed, pelletized, and heated at 1100 °C for 48 hr. CaTiO 3 perovskite was also prepared from CaCO 3 and TiO 2 by heating in air at 850–950 °C for 1 hr to decarbonate, and then held at 1700 °C for 48 hr. Both CaSiO 3 wollastonite and CaTiO 3 perovskite were confirmed to be single phase materials by powder x-ray diffraction and EPMA. The high-pressure experiments were performed using a 6–8 type multianvil apparatus at Gakushuin University. Tungsten car- bide anvils with truncated edge length of 5 mm were used through- out. A semi-sintered magnesia octahedron with edge length of 10 mm was used as a pressure transmitting medium. A cylindrical Pt heater of 2 mm diameter, thickness of 60 mm and 8.2 mm in length was used. A LaCrO 3 sleeve of inner diameter of 2 mm and outer diameter of 3.5 mm was placed outside of the Pt heater for thermal insulation, together with two LaCrO 3 end plugs of 2 mm in height. Atsushi Kubo  Toshihiro Suzuki  Masaki Akaogi ( ✉ ) Department of Chemistry, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171, Japan Fax: 81-3-59 92-10 29